When are offline caches automatically cleared? More settings would be nice

[zachary-foster]

Thanks for the great app!

The offline cache is a great feature, but I find that it is randomly cleared, which makes it unreliable. Its not clear to me what causes the cache to clear. It would be nice if I can customize when the cache is cleared. Perhaps the ability to specify how much space to use in the cache and then remove the oldest tiles as needed once it is filled?